This invention relates to a power steering device for an automotive vehicle.
In general, a power steering device is used to increase the supply of power fluid which assists the operation of a steering wheel at a low speed so that the steering wheel can rotate lightly. When a vehicle is running at a high speed, the power fluid is reduced to cause the steering wheel to rotate heavily so that the steering wheel can be stabilized.
Various types of power steering devices have been proposed. A prior art power steering device is to control the power fluid which is supplied from a steering pump to a power cylinder by means of an electromagnetic by-pass valve. Another type conventional power steering device is to control the power fluid by means of a fluid control valve in response to running or driving conditions of the vehicle.
However, in the conventional devices, within a high speed range, a majority of power fluid intermittently supplied by the steering pump in proportion to the engine speed is continuously discharged into an oil tank so that the power fluid is not effectively used, and energy saving cannot be attained.
Most conventional steering pumps for automotive vehicles are of an engine-driven type in which the speed of the steering pump is substantially the same as the engine speed. However, it is preferable that the supply of the power fluid is increased when a car is running at a low speed or stops. At an intermediate or high speed, the supply of the power fluid should be fully reduced or stopped, and the manual steering is preferable from the viewpoint of the steering stabilization.
For the purpose of energy saving, it has been proposed that the engine speed is reduced during idling and that the uppermost speed range of the engine is set very high so as to reduce the engine size and weight. In such a conventional engine-driven type steering pump, the steering assist power is insufficient at zero-speed steering, garaging or the like.